The present invention relates to the determination of the spatial distribution of electrons emerging from the surface of a radioactive body.
It more particularly applies to techniques employed in biology.
It is common practice to identify the location of molecules within a certain body or medium, by marking or labelling same with radioactive substances and by determining the position of the electrons emerging from the radioactive body or medium containing these molecules.
The most common technique used for determining this distribution is autoradiography which consists in placing a sample, such as a thin section containing the labelled molecules, against a photographic film, which has been exposed by the electrons (or .beta.-rays) emitted by the radioactive elements of these molecules.
The disadvantages of autoradiography are threefold:
an extremely long time (often several months) is required in order to be able to observe spots on the photographic film, for example in the case of sections of biological tissues;
an expensive apparatus is required in order to measure the photographic densities of the spots whenever quantitative informations are desired on the relative distributions of radiation intensity and therefore on the concentration of radioactive substances;
the accuracy as to the intensity of radiation is limited by the relatively reduced dynamic range of the photographic films, on the one hand, and by the relatively high background noise of same, on the other hand, particularly in the case of a very long exposure time, which is necessary as previously stated.
In return, autoradiography makes it possible to obtain a very high spatial accuracy, of about a few microns, which cannot be obtained by other conventional imaging processes, since the limitation in spatial accuracy solely results from the physical dimension of the distribution of the radioactive spot.
It has been proposed to accelerate the determination of the location of molecules labelled with radioactive elements within a body, by employing gas detectors which, in response to an electron emitted by the labelled molecules, produce an ionization giving rise to an electric pulse capable of being located, after having been submitted to an appropriate multiplication in the gas. As gas detectors, multiwire chambers have in particular been proposed.
The drawback of the gas detectors, in particular of the multiwire chambers, is the frequently considerable travel path of the electrons, emitted by the labelled (radioactive) molecules in the gas of the detector, which tends to limit the accuracy of the location of the labelled molecules. Actually, the travel at atmospheric pressure, may reach several hundreds of microns, when the molecules are labelled with tritium which emits low energy electrons or .beta.-rays (on an average at 6 KeV approximately), and tens of centimeters, when they are labelled with phosphorus 32 which emits high energy electrons (on an average at 600 keV approximately).
With a view of eliminating the aforesaid drawback of the gas detectors, Petersen, Charpak, Melchart and Sauli suggested, in Nuclear Instruments and Methods 176 (1980) p. 239-244, a gas detector in which the amplfication of the ionization electrons released in the detector gas is preferentially performed on electrons released in the vicinity of the detector surface against which the body to be examined and containing the labelled molecules is placed, thereby allowing an increased accuracy, about a few hundreds of microns, even in the case of electrons emitted by phosphorus-32, as can be seen from an article by Bateman, Stephenson and Connolly in Nuclear Instruments and Methods in Physics Research A 269 (1988) 415-424.
It should be noted that the gas detectors of the various above-mentioned types are provided with electrodes consisting in particular of wires, and that the location of the avalanches of multiplication, which are generated in the detector gas by the electrons originating from the radioactive body, is based on the detection of electrical pulses generated in these electrodes by the movement of the electrons and ions produced by the avalanches.
Contrary to the autoradiography approach on the one hand, and the gas detectors with the detection of the electric pulses produced in the electrodes, on the other hand, the present invention is based on the detection of the avalanches produced in a gaseous medium, in response to the electrons emitted by the radioactive body to be examined, by optical means which detect the ultraviolet or visible photons emitted by the avalanches.
On the other hand, an article of Charpak, Dominik, Fabre, Gaudean, Sauli and Suzuki in Nuclear Instruments and Mehods in Physics Research A 269 (1988), 142, discloses a detector implementing the optical detection of the avalanches produced by the electrons which are multiplied in an intense electrical field, these electrons being produced by ionization reactions, whereas the avalanches are produced between parallel grids between which a continuous high voltage is applied and the optical detection of the avalanches is made by a system which successively incorporates an image converter, an image intensifier and a CCD-type camera (charge coupled device camera), optically coupled to an image intensifier by optical fibres.
However, this last mentioned article in no way contemplates aPplying the gas detector described therein to the optical detection of the electrons emitted by radioactive bodies, and does not provide any teaching suggesting such an application.